There has heretofore been known a permanent magnet (PM) motor including a cylindrical stator having coils and a rotor which is disposed inside the stator, and which has permanent magnets buried therein. In the PM motor, the rotor is rotated by allowing currents to sequentially flow through a plurality of coils provided in the stator to cause mutual interaction between a rotating magnetic field generated in the coils and a magnetic field generated by the permanent magnets in the rotor. In such a PM motor, the number of revolutions is controlled according to a speed at which the coils for supplying the currents are sequentially switched.
Specifically, in the PM motor, induced electromotive force corresponding to the number of revolutions of the rotor is generated in the stator by the rotation of the rotor having the permanent magnets. This induced electromotive force is generated in a direction of canceling a voltage applied to the coils in the stator from outside. Thus, the maximum number of revolutions of the PM motor is limited to be not higher than a voltage to be applied to the coils from the outside by the induced voltage.
For example, in the conventional PM motor, a phase difference between an outer rotor and an inner rotor is controlled so as to enable reduction in the induced voltage by forming a path through which magnetic fluxes flow inside the rotor in high-speed rotation, and thus reducing the amount of magnetic fluxes flowing through the stator (see Japanese Patent Application Laid-Open Publication No. 2004-072978).